`createAnnotation` used to accept a partially filled-out annotation
object with arbitrary properties. However the argument was only ever
used to set the `$highlight` flag. Therefore we can simplify the
interface by making the method accept just a `highlight` flag instead.

 - Simplify `createAnnotation` argument

 - Add tests for handling of `highlight` option to `createAnnotation`

 - Remove the `createHighlight` method in guest as it was just a trivial
   wrapper around `createAnnotation({ highlight: true })`